1. Field of the Invention
The present invention relates to novel adsorbents for the purification of polyolefins and, more especially, to novel alumina-based adsorbents for the purification of polyolefins prepared by the polymerization of olefins in the presence of a system of coordination catalysts. This invention particularly relates to the purification of the reaction medium provided by the aforesaid polymerization process.
2. Description of the Prior Art
The polymerization of olefins is typically carried out in the presence of polymerization catalysts containing elements of Groups IVB, VB and VIB of the Periodic Table and, more particularly, containing vanadium, titanium and zirconium values. These catalysts also contain, as reducing agents, organometallic compounds (alkyl metallics), metallic hydrides or metal hydroxides. These catalysts are generally designated transition catalysts and have a high catalytic activity for the polymerization of olefins.
However, the polyolefins obtained are contaminated by metallic residues originating in the catalysts. It is thus strictly necessary to purify them prior to use, in order to prevent any coloration or degradation and to reduce their toxicity.
The processes for the polymerization of olefins generally entail apparatus for the recovery of the solvents and monomers contained in the polyolefins, such solvents and monomers being recycled back into the polymerization vessels. The presence of metals in these compounds promotes corrosion problems in the apparatus employed.
To purify the product polyolefin, it is known to this art to use different adsorbents, in particular the aluminas, for example.
Among the adsorbents used for such purpose, it is advantageous to utilize adsorbents in the form of pellets, which are handled more easily than powder or block articles of different shapes. In effect, a product in the form of pellets may be easily transported both for charging and discharging the purification columns, for example by use of various pneumatic systems.
From among such adsorbents, alumina is converted most readily into pellets. However, alumina, which is advantageous in view of its ease of handling and its good metal adsorption properties, presents a major disadvantage, i.e., it favors the isomerization of the solvents contained in the polyolefins, such as, for example, 1-butene.
Consequently, the solvents recovered cannot be completely recycled and most often they must be subjected to an additional purification to eliminate the isomerized products.